The present invention relates to a method and to a device for detecting an object with background suppression.
In particular, the term “device for background suppression” indicates an optoelectronic sensor that is able to detect the presence of an object inside an interval of set distances indicated, in general, by the term ‘field’.
In particular, the background suppression device is able to detect the presence of an object inside a pre-established distance interval comprised between the background suppression device and a set limit distance (also called cut-off distance), beyond which the presence of any object is ignored, which thus becomes part of the ‘background’. The background suppression device provides an information signal with which it is possible to associate alternatively a first logic or a second logic state indicating respectively the presence or the absence of an object within the cut-off distance.
The background suppression devices comprise a photoemitter that is suitable for emitting a light beam in a specific direction for detecting objects and photoreceiving means that is suitable for receiving at least partially, through the principle of optical triangulation, the light beam reflected by an object. Typically, these devices are used in the sector of industrial automation, of packaging machines or of the food industry, for example to check the presence of an object conveyed on conveyors or on conveyor belts, and are installed on one side of the conveyor or the belt to emit the light beam, at a frequency that is equal to the required sampling frequency, towards the conveyor. The cut-off distance is not in general pre-settable a priori when the device is manufactured, as it depends on the particular manner of use of the device. Thus each installation of the device is typically preceded by an optical adjustment procedure, by means of which an operator is able to adjust the device so that the cut-off distance is set. Beyond the cut-off distance the background starts, which may, for example, comprise a wall of the conveyor or the conveyor itself.
In detail, the photoemitter emits a light beam in the direction of the object, which is reflected by the object at an angle that depends on the distance of the object from the photoemitter, i.e. the nearer the object, the greater the reflection angle. The photoreceiving means makes a first and second analogue electric output signal available, the values of which, considered together, are in correlation with the presence of the object within the cut-off distance. Processing means receives the first and second signal and provides the information signal indicating the presence or the absence of the object.
Background suppression devices of mechanical or electronic type are known.
In background suppression devices of mechanical type, the photoreceiving means comprises two distinct adjacent photoreceivers: a first photoreceiver makes a first signal available and is arranged in such a manner as to receive light beams reflected by objects arranged as far as the cut-off distance; a second photoreceiver makes a second signal available and is arranged in such a manner as to receive beams reflected beyond the limit distance.
Consequently, by comparing together the first and the second signal, it is possible to obtain a differential information signal correlated to the presence of the object inside the field. In fact, if the object is present, the light reflected by the object impinges the first photoreceiver more than the second receiver, the first signal therefore has a greater amplitude than the second signal, and thus the background suppression device can provide an output information signal, the logic state of which indicates the presence of the object. On the other hand, if the light reflected by the object impinges the second photoreceiver more than the first photoreceiver, because light is mainly received beyond the limit distance, the first signal has a smaller amplitude than the second signal and the output signal is not activated. In other words, the sign of the difference between the two signals means that the output signal changes from a first to a second logic state or not, to indicate the presence of an object or only the reflection by the background.
In background suppression devices of electronic type photoreceiver means is used that consists of a PSD (Position Sensitive Device). The photoreceiver means has a semiconductor material layer provided at the ends with two electrodes through which the first and the second signal are made available.
The principle according to which the reflection angle of the light beam depends on the distance of the object from the photoemitter still remains valid, but the photoreceiver means, instead of comprising two distinct photoreceivers, exploits the property of the layer of semiconductor material of being sensitive to the position in which the layer is impinged by the reflected light beam. In this manner, the position in which the layer is impinged determines the value of the amplitude of the first and of the second signal and thus, by comparing the amplitude of the first and of the second signal, it is possible to provide a differential output information signal the logic state of which indicates the presence or absence of the object, in a similar way what has already been said for background suppression of mechanical type.
The background suppression devices disclosed above have the drawback of being imprecise in detecting objects that have zones of different colours or which are shiny. Switching between the aforesaid two logic states, i.e. the information on the presence/absence of the object inside the ‘field’ is in fact influenced by the reflective power of the object and the background suppression device may or may not detect an object, depending on the reflective power and/or the colour thereof.
For example, if an object has portions of two contrasting colours such as black and white or portions that are more or less reflective, the amplitude of the first and of the second signal mainly depends on the reflection of the white part, which is much more reflective than the black part, rather than the position of the object within the interval of set distances. The differential information signal, owing to the difference between the amplitude of the first and of the second signal, could indicate the absence of the object even if it is present. The background suppression device would be deceived.
A similar problem also occurs in the presence of a very reflective object that may spuriously give rise to reflected light beams, which, received by the background suppression device, are such as to deceive the background suppression device.
From U.S. Pat. No. 6,380,532, in order to resolve the problem of light beams reflected spuriously from shiny objects, making a background suppression device of mechanical type is known in which at least three distinct photoreceivers are associated with the photoemitter, in particular four distinct photoreceivers in a specific embodiment. The plurality of distinct photoreceivers are arranged adjacent on opposite sides to the photoemitter and are connected electrically together in a suitable manner to minimise the possibility that spurious reflections coming from the background or from shiny objects give rise to an incorrect information signal indicating the presence of an object.
The solution illustrated by U.S. Pat. No. 6,380,532 has the drawback that the mechanical complexity, the cost and the overall dimensions of the background suppression device are increased because of the increased number of photoreceivers present. In addition, the device illustrated by U.S. Pat. No. 6,380,532 does not completely resolve the problem of identifying the presence of a reflective object inside the limit distance.
In fact, in the case of an object made of reflective material, the light beam reflected therefrom can be directed outside the receiving means. In this case, as no photoreceiver of the background suppression device would receive reflected light, all the signals associated with the respective photoreceivers would coincide with or be equal to 0 and accordingly this situation would give rise to an output information signal the logic state of which would indicate the absence of the object.
The background suppression device would thus be deceived by the reflective object and would not be able to identify the presence thereof, albeit increasing the number of photoreceivers present.
The object of the present invention is to make a background suppression device that is free of the drawbacks disclosed above, in particular a device that is able to detect reflective objects.
According to the present invention, a background suppression device is made, as disclosed in claim 1.
According to the present invention, there is provided a method for detecting an object with background suppression as disclosed in claim 17.
Owing to the present invention, the background suppression device is able to identify an object, even if it is reflective inasmuch as, taking account of the first and of the second signal in a detecting condition and in a calibrating condition, the information signal indicating the presence or the absence of the object is not only directly a function of the fact that the reflected light beam impinges the first photoreceiver or the second receiver more but is also correlated with the first and the second signal stored in a set condition that is the calibrating condition.
In particular, according to one embodiment of the present invention, each variation on the first or the second signal in the detecting condition compared with the value of the respective first and second signal in the calibrating condition, a variation that is outside a respective permitted threshold interval, is interpreted as the presence of the object inside the cut-off distance. The calibrating condition corresponds to acquiring the first and the second signal in the absence of the object, i.e. to acquiring and storing the first and the second signal influenced only by the background. This accordingly enables an object to be identified as present even if the reflected light beam is directed by the object outside the receiving means, this condition being nevertheless different from the calibrating condition i.e. from acquisition of the background.